The present invention relates generally to the field of aphakic lenses. More specifically, the present invention relates to an aspheric silicone lens and a technique for fabricating a mold for making such lenses having virtually any surface contour, including non-symmetric surfaces. The invention also includes a technique for attaching and securing support members, or haptics, to such a lens after the lens has been formed and tested.
Intraocular lenses have been increasingly used in the last decade, in particular in aphakic patients after a cataract operation. Intraocular lenses provide many advantages over both spectacle and contact lenses. They permit a better elimination of perceptual problems and reduce image size disparity. Since the intraocular lens is intended to remain in situ, it eradicates the difficulties in inserting and removing contact lenses encountered by elderly patients. The use of an intraocular lens may also be advantageous for those working in unusual environments and for those whose visual requirements for occupation must be fulfilled. Presently, ophthalmologists and eye surgeons recommend that intraocular implant lens surgery be performed when the patient is not likely to manage a contact lens.
According to Norman S. Jaffe et al, Pseudothakos, published by The C.V. Mosby Company, 1978, the majority of patients who undergo lens implant surgery in the United States receive implants whose power is estimated from the basic refraction of the eye. Experience has shown however that there are many pitfalls in estimating the basic refraction in this way, in view of the high incidence of residual anisometropia and aniseikonia cases in patients thus corrected. More recently ophthalmologists surgeons have endeavoured to design bifocal intraocular lenses (IOL hereinafter) to focus both the near and far images on the retina. The Ocular Surgery News, Jun. 1, 1987, Volume 5, Number 11, reports the latest findings concerning bifocal IOL's. These IOL's, however, provide near and distance vision but do not provide a continuum in the dioptric range. To the best of the inventors' knowledge, there is not known any multifocal intraocular lens.
Intraocular lenses comprise some type of optical element and a support, or haptic, coupled thereto, for properly positioning and centering the intraocular lens within the eye. These lenses have typically included hard polymeric or glass optical elements with metallic or polymeric supports. During the past decade, the medical profession has made widespread use of intraocular lenses comprising polymethylmethacrylate (PMMA), a hard plastic composition. In general, PMMA lenses are cut on a precision lathe, using diamond cutters or injection molded, and then carefully post polished by a critical tumbling process in which the edges of the lenses are radiused and polished.
Recently, workers in the art have utilized lenses comprising a soft, biocompatible material, such as silicone. Silicone lenses have the advantage of being lighter in situ than PMMA lenses, and because they are flexible, they can be folded to reduce their, size during implantation into the eye in accordance with conventional surgical procedures. In the implementation of such a procedure, it is the desire of the ophthalmic surgeon to reduce to a minimum the amount of astigmatism and trauma induced in the eye. A technique known as phacoemulsification permits the removal of a diseased or damaged lens and the insertion of a new intraocular lens through an incision of as little as 3 to 4 millimeters. Unfortunately, this procedure is not compatible with the insertion of hard PMMA lenses, and surgeons have found it necessary to increase the length of the incision to at least 8 mm to insert such lenses, obviating at least one advantage of phacoemulsification technology. Methods of producing optical components, such as lenses, have not changed in principle in many years. The main requirements are that the optical surface be polished to a highly accurate shape. In the fabrication of a soft, biocompatible lens, a polished mold, in the shape required for the correct refraction of light for the material selected, is employed. Silicone elastomers, of medical grade, have been found ideally suited for this purpose. The uncured silicone polymer is introduced into the lens cavity of the mold, in an amount dictated by considerations relating to the lens size, refractive power, and structure; and allowed to cure, usually by heating the mold to 250.degree. to 350.degree. F. in a press. Several methods of molding the final lens have been employed and include injection molding, liquid injection molding, compression molding and transfer molding.
It is sometimes desirable to have a lens which includes plural regions having different spherical radii, an aspherical lens, or a lens having aspherical portions. A virtue of such lenses is that the various lens portions yield an increase in dioptric power as the radius of curvature decreases. A problem with making such lenses is the difficulty in obtaining a satisfactory mold of optical quality, having the desired changing radius of curvature. Currently, most molds are made using optical grinding or cutting equipment, or electrical discharge machining (EDM). The mold cavity is then post polished using standard optical lapping techniques. The resultant mold yields a lens having squared-off edges, which cannot be dramatically altered to provide a smooth, radiused edge without substantial risk of damaging the lens. Due to the size of the mold and the difficulties in obtaining an optical finish on a convex surface produced by such a mold, molds for intraocular lenses, having critically measured multiple radii or aspherical portions, using present techniques is very difficult to make and not cost effective. Thus, the present invention offers a method and apparatus for forming molds having such dissimilar shapes.
In another aspect of the present invention, a method of bonding haptics to the periphery of an intraocular lens is described. Haptic materials have included metal loops of various types, however, due to complications related to weight and fixation, such structures have proven undesirable. Presently, polypropylene is a preferred haptic material, although PMMA, nylon, polyamide, polyethylene, polysulfone, and great number of extruded plastics may be used as well. Polypropylene is very resistant to bonding to silicone. It is imperative that the haptics not become detached from the optical element after implantation, as this could have severe repercussions.
The current, preferred method for attaching haptics to the optical element of an intraocular lens is by way of a mechanical lock. This lock may be comprised of an anchor, or loop, through and around which the lens material is cured during the molding process of the lens. One problem associated with such a mechanical bonding technique is that the mechanical anchor often intrudes into the optical zone of the lens, adversely affecting the visual acuity of the patient. Problems also arise when the haptic material is heated to the molding temperature. In general, excessive heat causes the haptic material to become brittle and causes degradation of the material. In addition, the angle that the haptics make with the lens is often critical, ranging from between 0.degree. and 10.degree.. If the optical element is formed through and around the haptics, a separate mold would be required each time it was desired to change the angulation of the haptic. Further, proper angulation of the haptic with respect to the lens is very difficult to achieve during standard molding processes, as the introduction of the lens material into the mold cavity can cause the haptics to be slightly offset. In addition, the haptics tend to get smashed as the two halves of the mold are brought together and closed. Even if the haptic is properly secured to the lens, and able to withstand the molding temperatures and pressures, the lens must be optically tested and approved. A lens rejected for lack of optical quality would obviate the proper positioning and attachment of the haptics thereto. It would therefore be preferable to attach the haptics to the lens after the lens has been formed and optically tested, however, as mentioned above, the bonding of polypropylene to silicone has proven extremely difficult.
Therefore, there is a need in the art for a technique of making intraocular lenses having multiple radii portions or aspherical portions for providing varying degrees of dioptric power. Further, there is a need in the art for a method of attaching haptics to intraocular lenses in general, after the lens has been formed and optically tested.